Boron nitride has made news repeatedly in recent years as a material with an appealing mix of structural and physical properties. But it hasn’t made news as a catalyst, and certainly not one with the potential to drive global-scale industrial chemical processes.
That all just changed. Researchers have demonstrated that boron nitride (BN) selectively catalyzes conversion of propane to propene, a valuable chemical used worldwide on the multimillion-ton-per-year scale (Science 2016, DOI: 10.1126/science.aaf7885).
Researchers have mainly studied nanotube and one-atom-thick forms of BN that boast high strength, heat resistance, and unique electronic properties. A team at the University of Wisconsin, Madison, including Joseph T. Grant and Ive Hermans, have shown that BN unexpectedly works well as a catalyst that drives oxidative dehydrogenation of propane (ODHP). This process strips hydrogen from propane to form propene and oxidizes the hydrogen to form water.
Manufacturers typically produce propene by “cracking” large hydrocarbons in naphtha, a component of crude oil, with steam. But steam cracking plants have started to use shale gas instead of naphtha as a feedstock, a process that yields less propene. This has forced manufacturers to look for alternative ways to boost propene output.
Several dehydrogenation methods that do not use an oxidizer can drive the propane-to-propene reaction. But these methods gunk up the catalysts, shortening their lifetimes. Also, nonoxidative dehydrogenation and steam cracking are both highly endothermic, and therefore energy intensive.
In contrast, ODHP is exothermic and can run efficiently at hundreds of degrees lower than the other processes. According to industry estimates, a reaction running at relatively low temperatures such as these could reduce energy input by 45%. But the many ODHP catalysts that have been studied overoxidize propene, forming unwanted yet thermodynamically stable CO and CO2.
Not BN. The Wisconsin team finds that in the presence of oxygen, BN nanotubes and hexagonal-BN convert propane to propene and generate ethene, a valuable commodity, as the main by-product. Under one set of conditions, the reaction generated roughly 80% propene and 12% ethene.
“The discovery that boron nitride is not only a competent ODHP catalyst, but immediately takes the top position among all known catalysts, is absolutely remarkable,” says Henrique Teles, a senior research manager at BASF. “I would never have imagined that this material could be a competent catalyst for anything. I am positively sure, that this will open whole new avenues in catalysis.”
Nazeer A. Bhore, manager of breakthrough research at ExxonMobil, is also enthusiastic about the work. He notes that this discovery has the potential to reduce the cost and environmental footprint of propene production.